Red blood cells carry oxygen throughout the body and live for about 120 days. White blood cells help fight infection as part of the immune system. Platelets help the blood clot. Blood is made up of plasma, which allows cells to move, and four main cell types - red blood cells, white blood cells, platelets, and plasma. Arteries carry oxygenated blood away from the heart while veins carry deoxygenated blood back to the heart. Capillaries are where gas exchange occurs between blood and tissues. The immune system responds to infections through a coordinated response between white blood cells and antibodies.